This application claims priority under 35 USC 119 and contains subject matter related to Japanese Patent Application No. 11-202550 filed in the Japanese Patent Office on Jul. 16, 1999, the entire contents of which are hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to a sheet feeding device for use in an image forming apparatus such as a copying machine, a facsimile, a laser printer, or similar image forming apparatus.
2. Discussion of the Background
FIG. 5 is a schematic cross-sectional view illustrating a conventional sheet feeding device of an image forming apparatus, such as a laser printer. FIG. 6 is a cross-sectional view illustrating a part in the vicinity of a sheet feeding roller of the conventional sheet feeding device of FIG. 5. The image forming apparatus in FIG. 5 includes a sheet feeding roller 1, a sheet feeding cassette 2, a cassette bottom plate 3, a pair of sheet conveying rollers 4, sheets 5, an image forming section 6, a fixing section 7, a sheet discharging section 8, and a sheet discharging tray 9. The sheet feeding cassette 2 is configured so that it can be removed from the image forming apparatus by being drawn out to the right-hand side as seen in FIG. 5.
As illustrated in FIG. 6, the sheet feeding device includes a friction pad 10 at a lower portion of the sheet feeding roller 1 to separate a top sheet from the rest of the stack of sheets 5 in the sheet feeding cassette 2. The sheets 5 in the sheet feeding cassette 2 are fed through the conventional sheet feeding device via a friction pad sheet separating method. The friction pad 10 is attached to a friction pad pedestal 11 that is biased by a spring 12 to protrude upwardly. However, the friction pad pedestal 11 is prevented from protruding too far past a predetermined extent by a hook pawl 11a which is provided at a lower end portion of the friction pad pedestal 11 and is caught by a part of a holding section 2a of the sheet feeding cassette 2.
Operations of the sheet feeding device will now be described. First, the sheet feeding roller 1 rotates when a clutch (not shown) is turned on by a driving device (not shown), and thereby starts to feed the sheets 5. Then, a top sheet is separated from the rest of the sheets 5 in the sheet feeding cassette 2 by the friction pad 10 which is pressed by the sheet feeding roller 1. The top sheet is then conveyed to the sheet conveying rollers 4 which are disposed downstream of the sheet feeding roller 1 in the sheet conveying direction. After the leading edge of the top sheet reaches a nip portion between the sheet conveying rollers 4, the driving of the sheet feeding roller 1 is stopped when the clutch is turned off by the driving device. Subsequently, the sheet feeding roller 1 is rotated together with the top sheet conveyed by the sheet conveying rollers 4. When the trailing edge of the top sheet passes through a nip portion between the sheet feeding roller 1 and the friction pad 10, the rotation of the sheet feeding roller 1 is stopped.
In the above-described operations of the sheet feeding device, a load or force F is received by the top sheet of the sheets 5 when the top sheet of the sheets 5 is pulled out from the sheet feeding cassette 2 by the sheet conveying rollers 4 under the condition that the sheet feeding roller 1 is not driven. The load F is a sum of a load Fp and a load Ff and a load Fk. The load Fp is received by the top sheet of the sheets 5 when the cassette bottom plate 3 presses the top sheet of the sheets 5 against the sheet feeding roller 1. The load Ff is received by the top sheet of the sheets 5 when the top sheet of the sheets 5 is pressed against at the nip between the sheet feeding roller 1 and the friction pad 10. The load Fk is a sum of a load received by the top sheet of the sheets 5 when the top sheet of the sheets 5 is bent along a curved sheet conveying path, another load received by the top sheet of the sheets 5 when the sheet feeding roller 1 is rotated together with the top sheet of the sheets 5 conveyed by the sheet conveying rollers 4, and other loads.
In the above-described sheet feeding device, the load Fp is in a range of about 150 gf to 250 gf, the load Ff is in a range of about 250 gf to 400 gf, and the load Fk is in a range of about 50 gfxcx9c150 gf. Therefore, the load F is in a range of about 450 gfxcx9c800 gf. Because the sheet conveying rollers 4 pull out the top sheet of the sheets 5 from the sheet feeding cassette 2 against the above-described load, the following problems typically occur. First, the sheet conveying rollers 4 are likely to be worn due to heavy load. Second, if rubber of superior wear resistance is used for the sheet conveying rollers 4 so as to improve the wear resistance of the sheet conveying rollers 4, an increase in cost will result. Third, in order to convey the top sheet of the sheets 5 with a stable speed, it may be necessary to increase the sheet conveying force of the sheet conveying rollers 4. However, for this reason, a roller holding section for the sheet conveying rollers 4 may need to be of sturdy construction, and the load of the motor may increase. Thus, both an increased cost and size of the image forming apparatus may result.
In order to reduce the above-described load received by a sheet when the sheet is conveyed by the sheet conveying rollers 4, a conventional sheet feeding roller, having a cross-sectional shape which is approximately a partly cut-off circle (e.g., a semicircle), is known. As illustrated in FIGS. 7A through 7D, a sheet feeding roller 20 has a cross-sectional shape of a partly cut-off circle including an arc portion 22 and a chord portion 21. It is configured so that after one rotation of the sheet feeding roller 20, the sheet feeding roller 20 does not contact the friction pad 10 and the stack of sheets 5 in the sheet feeding cassette 2.
As illustrated in FIG. 7A, when the sheet feeding roller 20 is in a standby condition, the sheet feeding roller 20 is held such that the chord portion 21 of the sheet feeding roller 20 faces both the friction pad 10 and the stack of sheets 5 in the sheet feeding cassette 2. In this standby condition, the hook pawl 11a, which is provided at a lower end portion of the friction pad pedestal 11, is caught by the portion of the holding section 2a provided at the rear end of the sheet feeding cassette 2, so that the friction pad 10 is not elevated to a higher position. Thereby, a gap is formed between the upper surface of the friction pad 10 and the chord portion 21 of the sheet feeding roller 20. Moreover, when the sheet feeding roller 20 is in the standby condition, the cassette bottom plate 3 is locked by a locking mechanism (not shown), so that the sheets 5 in the sheet feeding cassette 2 are not elevated to a higher position. Thereby, the gap formed between the surface of the sheets 5 and the chord portion 21 of the sheet feeding roller 20 remains the same.
In the sheet feeding device including the sheet feeding roller 20 having a cross-sectional shape of a partly cut-off circle, when the feeding of the top sheet of the sheets 5 is begun, the sheet feeding roller 20 starts its one full rotation under the action of a one-rotation clutch (not shown). When the sheet feeding roller 20 starts to rotate, the cassette bottom plate 3 is unlocked by releasing the locking mechanism immediately before the arc portion 22 of the sheet feeding roller 20 contacts the top sheet of the sheets 5. Subsequently, the arc portion 22 contacts the top sheet of the sheets 5 in the sheet feeding cassette 2, and a few sheets of the sheets 5 start to be fed by the sheet feeding roller 20 as the sheets 5 are pressed against the sheet feeding roller 20 by a biasing force of a spring 23 serving as a bottom plate pressing mechanism. When the arc portion 22 of the sheet feeding roller 20 rotates to a position so as to contact the friction pad 10, the friction pad 10 is pressed against by the arc portion 22 and depressed to a predetermined position so as to be in pressing contact with the arc portion 22. Then, the top sheet is separated from the rest of the sheets 5 fed from the sheet feeding cassette 2 by the friction pad 10 and is conveyed downstream of the sheet feeding roller 20 as illustrated in FIG. 7B.
Subsequently, before the chord portion 21 of the sheet feeding roller 20 faces the sheets 5 in the sheet feeding cassette 2, the cassette bottom plate 3 is locked at a current position by the locking mechanism. Because, if the cassette bottom plate 3 is not locked before the chord portion 21 faces the sheets 5, the upper surfaces of the sheets 5 move up so as to contact the chord portion 21 and thus, no gap is formed between the upper surfaces of the sheets 5 and the sheet feeding roller 20.
As illustrated in FIG. 7C, the sheet feeding roller 20 further rotates and is held at a standby position wherein the chord portion 21 faces the sheets 5 and the friction pad 10. FIG. 7D illustrates the sheet feeding device in the condition that the height of the stack of sheets 5 in the sheet feeding cassette 2 is shorter than the height of the stack of sheets 5 in FIG. 7C. In FIG. 7D, the upper surfaces of the sheets 5 are held at a predetermined position when the upper surfaces of the sheets 5 contact the arc portion 22 of the sheet feeding roller 20. By locking the cassette bottom plate 3 with the locking mechanism while the sheets 5 contact the arc portion 22, the positions of the upper surfaces of the sheets 5, when the sheet feeding roller 20 is in the standby condition, can be kept constant regardless of the number of sheets 5 (or the height of the stack of sheets 5) in the sheet feeding cassette 2.
Referring to FIGS. 8A and 8B, the gap between the chord portion 21 of the sheet feeding roller 20 and the upper surface of the friction pad 10 will be described, when the sheet feeding roller 20 is in the standby condition. Although the sheet feeding roller 20, illustrated in FIGS. 8A and 8B, has a different shape than the sheet feeding roller 20, illustrated in FIGS. 7A through 7D, the operation and function of the sheet feeding roller 20 is substantially the same. Therefore, the structural elements of the sheet feeding roller 20 in FIGS. 8A and 8B having substantially the same functions as the structural elements in FIGS. 7A through 7D are designated with the same reference characters. In addition, although the shape of the holding section 2a, provided adjacent the sheet feeding cassette 2 for catching the hook pawl 11a at the lower end portion of the friction pad pedestal 11, is different from the holding section 2a in FIGS. 7A through 7D, there is no significant difference in operation and function.
FIG. 9 is a perspective view of the sheet feeding roller 20 illustrated in FIGS. 8A and 8B. The sheet feeding roller 20 is an integrally formed by molding such material as synthetic resin, plastic, etc. As illustrated in FIG. 9, the sheet feeding roller 20 is constructed of a partly cut-off cylindrical portion and two flange portions 30. The two flange portions are integrally formed with the above-described cylindrical portion at each end thereof. The cylindrical portion has the cross-sectional shape of a partly cut-off circle, including an arc portion and two chord portions. Each flange portion 30 also has the cross-sectional shape of a partly cut-off circle, including an arc portion and two chord portions. A belt-shaped member with a high coefficient of friction, such as the coefficient of friction of rubber, is put around the cylindrical portion and the belt-shaped member closely contacts the cylindrical portion.
Referring to FIGS. 8A and 8B, the sheet feeding roller 20 includes the arc portion 22 and chord portions 21a and 21b. The arc portion 22 contacts and feeds the sheets 5. The arc portion 22 is illustrated in FIGS. 8A and 8B by diagonal shading. The arc portion 22 corresponds to the arc portion of the cylinder portion and includes the surrounding belt-shaped member having the high coefficient of friction. The chord portions 21a and 21b correspond to the chord portions of the flange portion 30, illustrated in FIG. 9, and neither chord portion 21a nor chord portion 21b contact the sheets 5.
FIG. 8A illustrates a sheet feeding device when the sheet feeding roller 20 feeds the top sheet of the sheets 5, corresponding to the sheet feeding device illustrated in FIG. 7B. When the sheet feeding roller 20 feeds the top sheet of the sheets 5, the friction pad 10 is situated at a lower position than when the sheet feeding roller 20 is in the standby condition. The friction pad 10 is lower by a distance S1 because the arc portion 22 of the sheet feeding roller 20 contacts and presses the friction pad 10. When the sheet feeding roller 20 stops rotating and returns to the standby condition as illustrated in FIG. 8B, the friction pad 10 is situated at a more elevated position (i.e., by a distance S1) than when the sheet feeding roller 20 feeds the sheets 5. In this condition, as illustrated in FIG. 8B, a gap S2 is formed between the chord portion 21b and the upper surface of the friction pad 10. In the above-described sheet feeding device, the gap S2 is large enough to cause a plurality of sheets 5 to enter the gap S2. Consequently, a double feeding of the sheets (i.e., a plurality of sheets being fed at one time) is likely to occur when the top sheet is conveyed by the sheet conveying rollers 4. Specifically, when the coefficient of friction between the sheets 5 is large and when the sheets 5 have a property of attracting each other due to static electricity (e.g., a tracing paper), the sheets 5 under the top sheet are likely to be conveyed together with the top sheet when the top sheet is conveyed by the sheet conveying rollers 4.
A sheet separating method employing corner claws in a sheet feeding cassette is known to be effective in reducing the load received by a sheet when the sheet is pulled out from the sheet feeding cassette. Compared to a sheet separating method employing a friction pad, the sheet separating method employing corner claws generally has drawbacks. For example, double feeding of sheets is likely to occur, and the margin of the thickness of a sheet to be fed is limited to a smaller value (e.g., neither thick sheet nor thin sheet is suitable to be fed).
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these problems.
The preferred embodiments of the present invention provide a novel sheet feeding device and image forming apparatus, wherein a load received by a sheet can be reduced and double feeding of sheets can be prevented.
In order to achieve the above-described and other objectives, the present invention provides a novel sheet feeding device, including a sheet feeding cassette configured to accommodate a stack of sheets, and a sheet feeding roller that is provided downstream of the sheet feeding cassette in a sheet conveying direction to feed a top sheet of the stack of sheets in the sheet feeding cassette. A sheet feeding roller having a cross-sectional shape of a partly cut-off circle includes an arc portion and at least one chord portion. The sheet feeding device further includes a friction pad that is provided opposite the sheet feeding roller to separate the top sheet from the rest of the sheets in the stack in the sheet feeding cassette. A regulating member is provided on the sheet feeding roller so as to face the friction pad and to regulate a gap between the chord portion of the sheet feeding roller and an upper surface of the friction pad. When the sheet feeding roller is in a standby condition, the sheet feeding roller is held such that the chord portion of the sheet feeding roller faces the friction pad and the stack of sheets in the sheet feeding cassette at a predetermined distance apart therefrom. The friction pad is held at a more elevated position than when the friction pad abuts the arc portion of the sheet feeding roller, and a periphery of the regulating member is closer to the upper surface of the friction pad than the chord portion of the sheet feeding roller.
According to the present invention, the top sheet may slidably contact the regulating member when the top sheet is conveyed. The regulating member may include resin. The regulating member may rotate in a same direction as the sheet feeding roller.
According to another preferred embodiment of the present invention, a sheet feeding device includes a sheet feeding cassette configured to accommodate a stack of sheets, and a friction pad configured to separate a top sheet from the rest of the stack of sheets in the sheet feeding cassette. The sheet feeding device further includes a sheet feeding roller that is provided downstream of the sheet feeding cassette in a sheet conveying direction and opposite the friction pad to feed the top sheet of the stack of sheets in the sheet feeding cassette. The sheet feeding roller has a cross-sectional shape of a partly cut-off circle, including an arc portion and at least one chord portion. The chord portion includes a regulating portion that is formed as an integral part of the chord portion of the sheet feeding roller so as to face the friction pad and to regulate a gap between the chord portion and an upper surface of the friction pad when the sheet feeding roller is in a standby condition. When the sheet feeding roller is in the standby condition, the sheet feeding roller is held such that the chord portion of the sheet feeding roller faces the friction pad and the stack of sheets in the sheet feeding cassette a predetermined distance apart therefrom. The friction pad is held at a more elevated position than when the friction pad abuts the arc portion of the sheet feeding roller, and a periphery of the regulating portion is closer to the upper surface of the friction pad than the chord portion of the sheet feeding roller.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.